170 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



EXPLANATION OF PLATE IIL 



Fig. 14. — Youug Spanish mackerel tliivty-six hours after development began, and 

 twelve hours after it had left the egg. The yelk sack has begun to be 

 absorbed somewhat, and in consequence of its gradual collapse its wall is 

 retreating from the outer or epiblast layer so as to leave a larger cardiac 

 space in front and a similar space behind between the sack and the intes- 

 tine. The latter still retains the form of a much-flattened canal, which 

 is still occluded in the oesophageal and oral regions, where it is not yet 

 broken through as a mouth, though the branchial furrows have made their 

 appearance. The breast fin bf now occupies a more anterior position as a 

 vertical semicircular fold a little way behind the ear. The superficial 

 epiblast has been elevated from the brain so as to form a space above the 

 latter, developing the supracephalic sinus ss. X 50. 



Fig. 15. — Young Spanish mackerel forty-five hours from the beginning of development 

 and twenty-one hours after it has left the egg. The contents of the yelk- 

 sack have been mostly absorbed. The point where the mouth will appear 

 is indicated at m, behind which the Meckelian and branchial cartilages 

 are appearing. The heart is more developed, and exhibits an atrium, a 

 ventricle, and a bulbus arteriosus. The liver is appearing as a thickening 

 on the lower side of the fore-gut at Iv, while the hinder extremity of the 

 Wolffian duct is now plainly visible as a simple canal above the intestine, 

 but is widened behind the vertical portion of the hind gut into a urinary 

 bladder, al. The breast fin now occupies a vertical position and the rudi- 

 ment of the coraco-scapular arch or shoulder girdle has ajipeared in tho 

 pectoral fold at its base. A few colorless blood corpuscles have appeared 

 in the heart, but there is still no systemic circulation. X 50. 



